Answer 1 : Atomic mass is the sum of all protons and neutrons in the atomic nucleus.
Answer 2 : The term that describes elements with different atomic masses due to varying numbers of neutrons is "isotopes". Isotope elements have same number of protons but different number of neutrons.
Answer 3 : As stated in the answer 1, atomic mass of an atoms is the sum of all protons and neutrons in it's nucleus. So, this carbon isotope has 6 protons and 7 neutrons in its nucleus so : 7+6=13 . It's atomic mass is 13 atomic mass unit.(amu)
